1. Field of the Invention
The present invention relates to an imaging lens and an imaging apparatus, and particularly to an imaging lens that is appropriate as a lens for an in-vehicle camera, a camera for a cellular phone, a monitor camera or the like using an imaging device, such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor). Further, the present invention relates to an imaging apparatus including the imaging lens.
2. Description of the Related Art
Conventionally, imaging apparatuses, such as in-vehicle cameras, cameras for cellular phones, and monitor cameras, were known. The in-vehicle cameras, which are mounted in cars or the like, photograph the surrounding areas of the vehicles, and the cameras for the cellular phones are mounted on the cellular phones. The monitor cameras are installed to obtain monitor images and to prevent crimes. Generally, these imaging apparatuses include imaging lenses, and imaging devices, such as a CCD and a CMOS, which convert images formed by the imaging lenses into electric signals. The sizes of the imaging devices have been becoming smaller every year, while the resolutions of the imaging devices have been increasing. Consequently, the sizes of the imaging apparatuses including the imaging devices have been becoming smaller. Therefore, there is a growing demand for small and high-performance imaging lenses to be mounted on the imaging apparatuses. Meanwhile, the imaging lenses for the in-vehicle cameras and monitor cameras need to be produced at low cost, and to have high weather-tolerance characteristics, while achieving wide angles, for example, a full angle of view exceeding 180 degrees.
Imaging lenses used in the aforementioned fields are disclosed, for example, in Japanese Unexamined Patent Publication No. 2005-227426 (Patent Document 1), U.S. Pat. No. 7,471,465 (Patent Document 2), U.S. Pat. No. 7,298,561 (Patent Document 3), Japanese Unexamined Patent Publication No. 2008-122922 (Patent Document 4), and U.S. Pat. No. 5,933,286 (Patent Document 5). Patent Documents 1 through 4 disclose five-lens-structure imaging lenses, each including an aspheric lens. Patent Document 5 discloses a five-lens-structure imaging lens in which all of the five lenses are spherical lenses.
In recent years, the sizes of imaging devices became much smaller, while the resolution of the imaging devices became much higher. Therefore, there is a strong demand for small high-performance imaging lenses. Meanwhile, a demand for wider angle-of-view imaging lenses to be used in in-vehicle cameras and monitor cameras remains strong. However, the conventional technique was insufficient to structure a low-cost small lens system, while achieving a wide angle of view and high performance that can cope with high-resolution imaging devices, the resolutions of which became much higher in recent years.
In each of the five-lens-structure imaging lenses disclosed in Patent Documents 1 and 2, the Abbe number of a lens arranged on the most image side (the lens arranged closest to the image side among lenses of the imaging lens) is large. Therefore, it was difficult to correct the longitudinal chromatic aberration of the imaging lens in an excellent manner. In Patent Document 3, the imaging lens includes a cemented lens. Therefore, the imaging lens is inappropriate for use in tough conditions, such as in-vehicle use. Further, the cost is high. The imaging lens disclosed in Patent Document 4 uses many glass lenses. Therefore, the cost is high. In the imaging lens disclosed in Patent Document 5, all of the lenses are spherical lenses. Therefore, the performance of the imaging lens needs to be improved to cope with the increase of resolution in recent years.